Steam generator utilizing a recirculating system



Jun 1964 J. l. ARGERSINGER STEAM GENERATOR UTILIZiNG A RECIRCULATING SYSTEM mull.

II III] Filed July 2'7, 1961 .7

INPENTOR JOHN LARGERSINGER ATTORNEY United States Patent 3,135,250 STEAM GENERATOR UTILIZING A RECIRCULATIN G SYSTEM John I. Argersinger, Simsbury, Conn., assignor to Combustion Engineering, Inc., Windsor, Conn., a corporation of Delaware Filed July 27, 1961, Ser. No. 127,289 Claims. (Cl. 122-406) This invention relates to steam generators and particu Patented June 2, 1964 'ice . pipe 20, feedwater header 22, mixing header 24, pipe 26,

the turbine is only 'a small percentage of that required when operating atfull load. At the same time, a much higher percentage flow of fluid is required through the waterwalls of the furnace to prevent them from being damaged during start-up. One method of accomplishing this in a supercritical once-through steam generator is to provide a recirculating system around the waterwalls of the furnace, so that some of the fluid. flowing through the once-through flow system is recirculated back through the waterwalls again, thus maintaining the flow velocity through this section of the through-flow system suflicient- 1y high to prevent the tubes from being damaged.

It is an object of this invention to provide a oncethrough supercritical steam generator utilizing a recirculating system, whereby the recirculating pump is located in the once-through flow system, in series with the feedwater pump.

It is another object of this invention to provide a reserve pump in parallel with the recirculating pump, which can be brought into operation in the event the main recirculating pump should not operate properly.

Other and further objects of the invention will become apparent to those skilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises an arrangement, construction and combination of the elements of the inventive organization in such a manner as to attain the results desired as hereinafter more particularly set forth in the following detailed description of the illustrative embodiment, said embodiment being shown by the accompanying drawing wherein:

FIGURE 1 shows a once-through steam generator utilizing a recirculating system and incorporating the novel features of my invention; and

FIGURE 2 is a horizontal cross section taken at line 2-2 of FIGURE 1.

inlet header 27, centerwall tubes 28, outlet header 29, pipe 30, inlet header 32, waterwall tubes 34, outlet header 36, and heat exchange members 38 and 40 which are conventionally referred to as the primary and final superheaters, respectively, in a subcritical boiler. The fluid leaving heat exchange member 40 by way of pipe 42 is used in driving the turbine 46.

When the turbine is first started up, only a small amount of fluid is allowed to flow through the turbine, this amount being but a small percentage of that required when operating at full load. Yet in order to protect the centerwall tubes 28, and the waterwall tubes 34 which line substantially all four walls of the furnace, a much larger flow is necessary, in order to maintain a predetermined minimum velocity.. If this high velocity flow is not maintained through the tubes 28 and 34, the tube metal will not be adequately cooled and the tubes will be damaged or ruptured by overheating from the combustion gases.

In order to allow a high velocity flow through these tubes and still just supply a small amount of fluid to the turbine, a circulating system is added to the once-through flow system, which recirculates fluid leaving the outlet of waterwall tubes 34 back to the inlet of centerwall tubes 28.

The circulating system comprises pipe 48 which extends from the outlet header 36to mixing header 24. Pipe 48 contains check valve 50, the purpose of which is to prevent reverse flow through pipe 48. Recirculating pump 52, which is preferably a centrifugal pump although other types could also be used, maintains flow through the recirculating system when the steam generator is first being started up, and also when it is operating below a predetermined percentage of its full load capacity. Pump 52 is driven by motor 53, which is activated and deactivated by switch 51.

Reserve pump 60is provided in parallel with pump 52 to handle the flow when the main recirculating pump 52 is not operating properly. If maintenance or replacement of pump 52 is necessary, motor actuated gate valve 54, located upstream of pump 52 can be closed, motor actuated gate valve 62 in bypass pipe 58 can be opened, and switch 59 closed to actuate motor 61, thus putting reserve pump 60 into operation. Check valves 56 and 64 prevent reverse flow through pumps 52 and 60, respectively.

Looking now to FIGURE 1, 10 is'a once-through I boiler operating at supercritical pressure, or pressure above 3206.2 p.s.i.a. Fuel burning means 12 are provided for the furnace 14, and the hot combustion gases flow through the furnace and associated gas pass 16, thus heating the fluid flowing through the various. heat exchange members, which fluid will ultimately be used to drive a turbine 46 to generate electricity.

Fluid at approximately 3500 p.s.i. is supplied to the through-flow system by feedpump 17. The feedpump 17, at full load, should supply the through-flow system with fluid at a pressure somewhat more than 3500 p.s.i., so that the ultimate pressure of the steam leaving the boiler circuit will be at 3500 p.s.i., afterthe pressure losses in- The through-flow system'comprises an economizer .18,

The recirculating pump 52 should be of such capacity that it will insure a minimum velocity flow, for example three feet per second, through the tubes 28 and 34 at all times. As the turbine 46 is gradually brought up to full load by opening valve 44 more, a point will be reached at which the flow through the once-through system will be suflicient to. protect the tubes 28 and 34 within furnace 14. At this time the recirculating pump is not required, and may be de-energized if desired. In one installation, this point was determined to be of full load. However, the percent required may vary between installations, depending on such factors as the size of the tubes lining the furnace walls, the tube material, the temperature of the combustion gases within the furnace, etc. In most installations today once a turbine has been brought up to full load, it is desired to later beable to run it at some load less than full load capacity, such as 10%, 50%, or 70% of full load. Obviously, when the turbine is brought down below the 70% load range, then the recirculating system must again be utilized to maintain the minimum flow velocity through the vapor generating section, or tubes 28 and 34 within the furnace,

The operation of the system will now be described. When it is desired to start up the unit, feedwater pump 17 is energized, and forces cold water through the economizer, centerwall tubes, waterwall tubes, and other sections of the once-through flow system. The recirculating pump 52 is then started up, and induces flow through the recirculating system. Fuel is then supplied to the fuel burning means and ignited.

The fluid in the system will gradually increase in temperature, and in pressure, until eventually steam will be created, and also the pressure will be above critical pressure, at approximately 3500 p.s.i., which is the pressure the unit has been designed to operate at. It is desirable to operate at pressures above critical, since the single phase fluid at this pressure continuously changes temperature in its transition from Water heating to steam superheating and at no single point in this type of steam generation are there fluids with simultaneously difiering densities such as exists at subcritical pressure, or below 3206.2 p.s.i.a. If the unit were operated at subcritical pressure, then when a small amount of the liquid in the system started turning to steam, the recirculating pump 52 would not becapable of pumping this mixture. When pockets of steam passed through the pump, the steam would be substantially compressed, turning it back into the liquid state, making the pump operate erratically and also possibly causing damage thereto. This problem is not en countered when the fluid is above the supercritical pressure, and regardless of the temperature, the pump can handle it. My invention could, however, be used on a subcritical unit, if a separating drum were installed in the recirculating lines. The steam could then be separated out, and only water allowed to pass through the recirculating pump.

As steam is formed in the system, the valve 44 up stream of the turbine 46 is cracked open, and a very small percentage of fluid is allowed to flow through the turbine. This percent is gradually increased by opening valve 44 more as the turbine warms up. The turbine must be slowly warmed up to prevent undue stresses and strains from being created therein by allowing a large amount of extremely high temperature steam to pass therethrough instantaneously. The time required to warm up a turbine can be six hours or longer.

During this start-up period, a portion of the fluid flowing in the through-flow circuit is withdrawn through the recirculating system, induced by means of recirculating pump 52, and recirculated back through tubes 28 and 34 again. As steam is generated in the circuit, and more and more of it is allowed to flow to the turbine during start-up, the pressure loss in the through-flow circuit from header 24 to header 36 gradually approaches and finally exceeds the pressure head of the recirculating pump. It is a well-known principle that as fluid flow increases through a pipe of a given dimension, the pressure loss of the fluid flowing therethrough increases accordingly. This condition of the pressure loss in the through-flow circuit from header 24 to header 36 (considering the pressure loss through recirculating line 48 to be negligible) exceeding the pressure head of the recirculating pump arises when the steam generator .is operating at approximately 70% of its full load capacity, and above this point there is no flow through the recirculating system. The pump 52 is designed to have a capacity sufficient to maintain flow through the recirculating system until the steam generator is operating at 70% load. The check valve 50 prevents reverse flow through pipe 48 when the unit is operating above 70%.

An example of operating pressures for a once-through unit containing a recirculating system in accordance with this invention might be as follows: fluid would leave the once-through circuit at 3500 p.s.i. The feedpump would supply fluid at approximately 3500 p.s.i. when the unit is operating at 70 percent load or less. The recirculating pump positioned in the once-through circuit would have a capacity capable of adding 100 p.s.i. pressure to the fluid passing therethrough. Then the boiler circuit would be designed such that when the unit operating at 70 percent load the pressure loss of the fluid flowing through the circuit would be 10Q p.s.i. Thus recirculation through the recirculating system will stop at this 70 percent load point. Under these operating conditions, the total pressure loss when operating the unit at full load, or percent load, might be p.s.i. Thus the feedpump should be capable of supplying fluid at 3650 p.s.i. to the inlet of the once-through circuit, in order to have the fluid leaving-the once-through circuit at 3500 p.s.i.

When 70% is reached, the recirculating pump 52 may be deactivated by opening switch 51, and the pump will then float or idle in the line. With the pump deenergized, it will act as a restriction in the through-flow system above 70%. In order to minimize this problem, motor-actuated gate valve 62 in bypass pipe 58 can be opened at this time, if it is not already open, and a portion of the flow can then be allowed to pass through this pipe. Gate valve 62 can either be opened manually by the operator by energizing its motor, or it could be automatically opened when the unit reaches 70% load. Any suitable control which is related to and varies directly with the load on the unit could'be used, such as the fuel feed input control, or the feedwater flow control. As illustrated, the pressure drop across restriction 80 is measured by control unit 82, which pressure drop is indicative of steam flow and thus proportional to the load on the steam generator. This control is used to deactivate pump 52 and open motor-actuated gate valve 62, when 70% full load is reached.

Reservepump 60 also can be put into operation in the event the main recirculating pump 52 fails. If this happens, gate valve 54 is closed, gate valve 62 is opened, and switch 59 is closed to energize pump 60. Recirculating pump 52 may then be isolated from the system by gate valve 54 andcheck valve 56, and can be repaired or replaced, as required.

Most utility boilers today are designed to operate over a wide load range, for example from 10-50% on up to full load. When the unit is brought down from full load to below the 70% point, the recirculating system is needed again, and recirculating pump 52 must be actuated at this time. 7

It will be seen from the following that the reserve pump 60 and its associated bypass pipe 58 has a threefold purpose. It can be put into use, thus permitting continued operation of the boiler, in the event the main recirculating pump 52 fails; it permits repair or replacement of the main recirculating pump 52 with the boiler in operation; and the bypass pipe 58 can be used to handle a portion of the flow when operating in excess of 70% after the pump 52 has been deactivated, so that it does not act as a substantial restriction in the throughflow circuit.

Although the parallel pump arrangement has been illustrated as located upstream of the centerwall tubes 28, it could also be used in other locations in the throughflow system, such as downstream of the centerwall tubes 28, or downstream of the Waterwall tubes 34, as illustrated in FIGURES 12 and 13 of the commonly assigned application of W. W. Schroedter application, Serial No. 127,395, now Patent No. 3,135,252 of June 2, 1964, and entitled Recirculating System for Steam Generator.

As illustrated, the recirculating pump receives fluid from mixing header 24. After the unit has been running for a time, the fluid in the recirculating system will be hotter than that coming from the economizer, and the mixing header 24 is for the purpose of mixing these two fluids, so that a single fluid of uniform temperature flows to pump 52 at all times.

While I have illustrated and described the preferred embodiment of my invention, it is to be understood that such is merely illustrative and not restrictive and that variations may be made therein without departing from the spirit and scope of the invention. I therefore do not wish to be limited to the precise details set forth but desire to avail myself of such changes as fall within the purview of my invention.

What I claim is:

1. In a once-through steam generator, a through-flow system supplying fluid to a turbine, a furnace, fuel burning means associated with said furnace, part of said through-flow system residing in said furnace, a feedpump for supplying fluid to said through-flow system, a recirculating system around a portion of said through-flow system so that some of the fluid flowing in said through-flow system can be withdrawn through said recirculating system and made to flow through said portion again, a first pump located in said portion of the through-flow system, said first pump having a limited capacity which is just sufficient to maintain flow through said recirculaing system until 1 the steam generator is operating at a predetermined percentage of full load capacity, and above said predetermined percentage said first pump acts as a restriction in the through-flow system, means for automatically deactivating said first pump when the steam generator reaches said predetermined percentage, a bypass pipe around said first pump, and a second pump located in said bypass pipe.

2. In a once-through steam generator, a through-flow system, a furnace, fuel burning means associated with said furnace, part of said through-flow system residing in said furnace, a feedpump for supplying fluid to said throughflow system, a recirculating system around a portion of said through-flow system, so that some of the fluid flowing in said through-flow system can be withdrawn through said recirculaing system and made to flow through said portion again when the steam generator is operating below a predetermined percentage of its full load capacity, a first pump located in said portion of the through-flow system, said first pump having such a limited capacity that it just maintains flow through said recirculating system until said predetermined percentage is reached, a first valve in said portion to prevent reverse flow therethrough, a bypass pipe having an inlet connected to said portion upstream of said first pump and first valve, the outlet of said bypass pipe connected to said portion downstream of said first pump and valve,v a second pump located in said bypass pipe, a second valve located in said bypass pipe to prevent reverse flow therethrough, a third valve located in said bypass pipe between said bypass pipe inlet and said second pump, and means responsive to load conditions for deactivating said first pump and opening said third valve when the steam generator reaches said predetermined percentage of its full load capacity, whereby when the steam generator is operating above said predetermined percentage, some fluid flows through said bypass pipe.

3. In a supercritical once-through steam generator, at through-flow system, a furnace, fuel burning means associated with said furnace, part of said through-flow system residing in said furnace, a feedpump for supplying fluid to said through-flow system, a recirculating system around a portion of said through-flow system, so that some of the fluid flowing in said through-flow system can be Withdrawn through said recirculating system and made to flow through said portion again when the steam generator is operating below a predetermined percentage of its full load capacity, a centrifugal pump located in said portion of-the through-flow system, said centrifugal pump having such a limited capacity that it just maintains flow through said recirculating system until said predetermined percentage is reached, and above said predetermined percentage it acts as a restriction in the through-flow system, a

first valve in said portion upstream of said centrifugal pump, a second valve in said portion downstream of said to said portion upstream of said first valve, the outlet of said bypass pipe connected to said portion downstream of said second valve, a second pump located in said bypass pipe, a third valve located in said bypass pipe upstream of said second pump, and a fourth valve located in said bypass pipe downstream of said second pump, and means responsive to load conditions for deactivating saidfirst pump and opening said fourth valve when the steam generator reaches said predetermined percentage.

4. In a vapor generator having a through-flow circuit comprised of a number of heat exchange sections in series flow relation with the through-flow being heated to its desired temperature upon traversal of said several sections the method of operation comprising introducing suflicient flow of the working medium through said circuit to satisfy the load on the generator by pumping said working medium into said circuit at a zone at the beginning thereof, establishing a recirculation of the working medium through a portion of said system by a limited pumping action produced in the through-flow system at a zone downstream of the first-mentioned zone, maintaining this pumping action below a predetermined load on the generator to maintain the recirculation below said load, terminating said pumping action above said load while continuing to pass at least a portion of thethrough-flow through the second mentioned zone, bypassing the remainder of the through-flow around the second-mentioned zone.

5. In a once-through flow vapor generator having a through-flow circuit, feedpump means connected into the upstream portion of said circuit, a recirculating system superimposed upon the through-flow circuit for recirculating a working medium through a portion of the throughflow circuit and including a bypass conduit connected around said portion, recirculating pump means connected into the through-flow circuit intermediate the ends of said bypass conduit so that the through-flow must pass therethrough, said recirculating pump means including a pair of pumps connected in parallel and having check valves associated therewith to prevent reverse flow therethrough, and means for selectively activating one or the other of said pumps, the method of operation comprising establishing by the said feedpump means sufiicient flow of the working medium through the through-flow circuit to meet the demand on the generator, activating one of said pumps in parallel and eliecting suflicient pumping action thereby to cause a recirculation of working medium through said portion of the through-flow circuit at and below a predetermined load but above said predetermined load the pumping action being incapable of effecting such recirculation and with the pump then acting as a restriction in the through-flow circuit, simultaneously forcing the working medium, by means of said feedpump means, through the two parallel connected pumps at loads on the vapor generator above said predetermined load, and deactivating said one of said pumps in parallel when said predetermined load is reached.

References Cited in the file of this patent UNITED STATES PATENTS 2,255,612 Dickey Sept. 9, 1941 2,969,048 Buri Jan. 24,1961

FOREIGN PATENTS 509,746 Great Britain July 20, 1939 831,175 Great Britain Mar. 23, 1960 

1. IN A ONCE-THROUGH STEAM GENERATOR, A THROUGH-FLOW SYSTEM SUPPLYING FLUID TO A TURBINE, A FURNACE, FUEL BURNING MEANS ASSOCIATED WITH SAID FURNACE, PART OF SAID THROUGH-FLOW SYSTEM RESIDING IN SAID FURNACE, A FEEDPUMP FOR SUPPLYING FLUID TO SAID THROUGH-FLOW SYSTEM, A RECIRCULATING SYSTEM AROUND A PORTION OF SAID THROUGH-FLOW SYSTEM SO THAT SOME OF THE FLUID FLOWING IN SAID THROUGH-FLOW SYSTEM CAN BE WITHDRAWN THROUGH SAID RECIRCULATING SYSTEM AND MADE TO FLOW THROUGH SAID PORTION AGAIN, A FIRST PUMP LOCATED IN SAID PORTION OF THE THROUGH-FLOW SYSTEM, SAID FIRST PUMP HAVING A LIMITED CAPACITY WHICH IS JUST SUFFICIENT TO MAINTAIN FLOW THROUGH SAID RECIRCULATING SYSTEM UNTIL THE STEAM GENERATOR IS OPERATING AT A PREDETERMINED PERCENTAGE OF FULL LOAD CAPACITY, AND ABOVE SAID PREDETERMINED PERCENTAGE SAID FIRST PUMP ACTS AS A RESTRICTION IN THE THROUGH-FLOW SYSTEM, MEANS FOR AUTOMATICALLY DEACTI- 